Relative Dating of Rocks
Youngest layer of rock
Relative Dating and the
Law of Superposition
Relative dating is determining
the age of a rock or geologic
event when comparing it to
other rocks or geologic events.
The Law of Superposition
states that
Oldest layer of rock
“…in an area of
undisturbed strata, the
bottom layer would be the
oldest (since it was the first
to form). Each layer above
the bottom layer would be
progressively younger.
The top layer of exposed
strata would be the
youngest (since it was the
last to form).”
Exceptions to the Law
of Superposition
Much of the rock strata observed on
Earth’s surface has been impacted by
the crustal dynamics of the Earth.
“How can a scientist determine the
relative age of rock layers under
these conditions?”
Relative Dating of undisturbed rock?
EASY!!!!!
Relative Dating of disturbed rock?
MORE DIFFICULT!!!!!
Dating Rock Features
An igneous intrusion in
younger than the rocks it
cuts through
“A” is younger than B, C,
and D
An igneous extrusion in
younger than the rocks it
cuts through
“D” is younger than
E and F
Dating Rock Features
An unconformity is a
buried weathered or
erosional surface. An
unconformity is younger
than the disturbed rock
layer.
Layer J was deposited and lithified
first. Layer J was weathered and/or
eroded (unconformity). Lastly, Layer
K was deposited and lithified.
An inclusion is a portion of
rock that is within an igneous
intrusion. For various
reasons, the portion of rock
did not melt and would be
older than the intrusion it is
within.
The rock fragments with intrusion “M”
are older than “M”.
Layers Q, O, N. M and L were originally deposited
The event of
as horizontal layers. These layers were then tilted. deformation of strata is
The tilting of the strata is younger than the formation
younger than the
of the strata.
strata itself.
Relative Dating – Sample #1
Determine the relative age of
the rock and geologic events
from the strata seen below.
Arrange the strata from
youngest to oldest.
Relative Dating – Sample #2
Determine the relative age of the rock and
geologic events from the strata seen below.
Arrange the strata from youngest to oldest.